We have developed a detailed mathematical model for Ca2+ handling and ionic currents in the rabbit ventricular myocyte. The objective was to develop a model that: 1), accurately reflects Ca-dependent Ca release; 2), uses realistic parameters, particularly those that concern Ca transport from the cytosol; 3), comes to steady state; 4), simulates basic excitation-contraction coupling phenomena; and 5), runs on a normal desktop computer. The model includes the following novel features: 1), the addition of a subsarcolemmal compartment to the other two commonly formulated cytosolic compartments (junctional and bulk) because ion channels in the membrane sense ion concentrations that differ from bulk; 2), the use of realistic cytosolic Ca buffering parameters; 3), a reversible sarcoplasmic reticulum (SR) Ca pump; 4), a scheme for Na-Ca exchange transport that is [Na]i dependent and allosterically regulated by [Ca]i; and 5), a practical model of SR Ca release including both inactivation/adaptation and SR Ca load dependence. The data describe normal electrical activity and Ca handling characteristics of the cardiac myocyte and the SR Ca load dependence of these processes. The model includes a realistic balance of Ca removal mechanisms (e.g., SR Ca pump versus Na-Ca exchange), and the phenomena of rest decay and frequency-dependent inotropy. A particular emphasis is placed upon reproducing the nonlinear dependence of gain and fractional SR Ca release upon SR Ca load. We conclude that this model is more robust than many previously existing models and reproduces many experimental results using parameters based largely on experimental measurements in myocytes.

Rationale Increased contractility of arterial myocytes and enhanced vascular tone during hyperglycemia and diabetes may arise from impaired large conductance Ca2+-activated K+ (BKCa) channel function. The scaffolding protein AKAP150 is a key regulator of calcineurin (CaN), a phosphatase known to modulate expression of the regulatory BKCa β1 subunit. Whether AKAP150 mediates BKCa channel suppression during hyperglycemia and diabetes is unknown. Objective To test the hypothesis that AKAP150-dependent CaN signaling mediates BKCa β1 downregulation and impaired vascular BKCa channel function during hyperglycemia and diabetes. Methods and Results We found that AKAP150 is an important determinant of BKCa channel remodeling, CaN/NFATc3 activation, and resistance artery constriction in hyperglycemic animals on high fat diet (HFD). Genetic ablation of AKAP150 protected against these alterations, including augmented vasoconstriction. D-glucose-dependent suppression of BKCa channel β1 subunits required Ca2+ influx via voltage-gated L-type Ca2+ channels and mobilization of a CaN/NFATc3 signaling pathway. Remarkably, HFD mice expressing a mutant AKAP150 unable to anchor CaN resisted activation of NFATc3 and downregulation of BKCa β1 subunits, and attenuated HFD-induced elevation in arterial blood pressure. Conclusions Our results support a model whereby subcellular anchoring of CaN by AKAP150 is a key molecular determinant of vascular BKCa channel remodeling, which contributes to vasoconstriction during diabetes.